A proximity detector is used in a wireless telephone handset to detect whether the handset is close to the ear of the user, in order to switch the handset between loudspeaking and handset mode of operation. The proximity detector ensures that the device will not switch to the loudspeaking mode while it is close to the user's ear, thereby safeguarding the hearing of the user from damage caused by high volume sound.
However, the proximity detector can fail, and it is desirable to have it cause the device to fail to a permanent handset mode, rather than to a loudspeaking mode. U.S. Pat. No. 5,224,151, issued Jun. 29, 1993 to Bowen et al describes a proximity detector which uses the reflection of infrared light from the ear of the user to determine proximity. An infrared light emitting device transmits light toward the ear of the user, which is reflected from the ear of the user, the reflected light being received by an infrared receiver. In the absence of receipt of reflected light, an electronic circuit connected to the infrared receiver determines that the device should be in the loudspeaking mode, and switches it into the loudspeaking mode. If the infrared receiver detects reflected light, the electronic circuit concludes that the device should be in the handset mode, and switches it into the handset mode. The device can be characterized as providing a ranging function.
However, if the light emitting device fails, and the handset is used close to the ear of the user, the electronic circuit concludes that there is no reflected light, and causes the device to switch into the loudspeaking mode, creating a danger to the hearing of the user.
To avoid the danger, the aforenoted patent describes the use of a safeguarding structure, in which another infrared transmitter transmits a light signal to the infrared receiver. The signals from the main infrared transmitter and the safeguarding transmitter are modulated. The electronic circuit detects the modulated signals, and through logical processing of the signals determines whether the ranging function provided by the main transmitter--infrared receiver--circuit is operating properly.
However if the main transmitter fails, or if any of the associated drive circuitry fails, the system does not fail safe, since it does not check whether the main infrared transmitter is operating. The result can be operation at high volume, with resulting damage to the user's ear.
In addition, the extra electronic circuitry required to provide the safeguarding function is clearly expensive, particularly when large volumes of devices are to be manufactured, and in an industry where severe price and functionality competition occurs.